1. Field of the Invention
The present invention relates to a method for treating magnesium alloys and particularly, to a method for treating magnesium alloys by non-chromate chemical conversion.
2. Description of Prior Art
As one of the most popular materials, magnesium alloys are environmentally friendly, lighter (only ⅔ of aluminum""s specific weight), better in heat transfer (51 w/mxc2x7k of thermal conductivity), stronger, and exhibit excellent ability in shielding electromagnetic interrupt (EMI). Additionally, raw magnesium is abundant, composing 2.5% of the earth""s crust and 0.13% of the earth""s bodies of water. Therefore, magnesium alloys have replaced some plastic materials and have been widely applied to the 3C industries (computer, communication and consumption electronics productions) and appliances. However, magnesium alloys corrode easily when exposed to atmosphere. Therefore, additional surface treatment is required. Currently, the chemical conversion process improves corrosion resistance and paint adhesion of the magnesium alloys. In general, the surface film after the chemical conversion process is quite thin, having thickness less than 5 xcexcm, so that this treatment causes no appreciable dimensional changes to the parts.
One conventional surface treatment of magnesium alloys is disclosed in U.S. Pat. No. 4,676,842, in which chromate is used in acid pickling and chemical conversion. Four chemical conversion processes developed by R. W. Murray and J. E. Hills include mechanical polishing, solvent cleaning, alkali cleaning and acid pickling, as described in xe2x80x9cMagnesium Finishing: Chemical Treatment and Coating Practicesxe2x80x9d. These processes can be applied individually or in combination, depending on whether the magnesium alloy was manufactured by casting or rolling. Similarly, chromate is used in acid pickling in the above processes. Scales formed during heating can be treated with a mixture of nitric acid and acetic acid. U.S. Pat. No. 2,302,939 also mentioned a method for removing oxides, in which organic acids such as tartaric acid, citric acid and acetic acid, are used and show better results than inorganic acids. The magnesium alloys are then treated with nitric acid-chromate to form corrosion-resistant surface films thereon.
The casting process produces more complex impurities than the rolling process, such as oxides or hydroxides of magnesium alloys, aluminum alloys, etc. and smuts and/or scales generated by grease or carbonates, and are removed with difficulty. Strong acids and alkalis are thus needed.
Using chromate in conventional processes prevents corrosion and conserves a proper appearance. Unfortunately, chromate will be forbidden because of its toxicity to human beings and the environment. Therefore, it""s necessary to develop an alternative surface treatment for magnesium alloys to minimize pollution.
The object of the present invention is to provide a method for treating magnesium alloys, which generates less pollutants by a non-chromate chemical conversion process and is suitable for the magnesium alloys manufactured by casting or rolling.
Another object of the invention is to provide a method for treating magnesium alloys, which can improve corrosion resistance and paint adhesion of magnesium alloys.
A further object of the invention is to provide a method for treating magnesium alloys, which can produce an admirable appearance.
In order to achieve the above objects, the method of the present invention primarily includes steps (1) providing a magnesium alloy having at least one surface for treatment; (2) cleaning the surface of the magnesium alloy with an alkali cleaner; (3) rinsing the magnesium alloy with water to eliminate the alkali cleaner; (4) pickling the magnesium alloy with acid cleaner to eliminate oxides and metal impurities on the surface; (5) treating the surface of the magnesium alloy with a chemical conversion processing agent to form a surface film having corrosion resistance and color protection properties; and (6) treating the surface with a sealing agent to reinforce corrosion resistance and paint adhesion of the surface film.
In the present invention, non-chromate chemicals used in acid pickling and chemical conversion processes can decrease pollution that frequently occurs in the conventional processes. Additionally, Ti or Si coupling agents of the present invention facilitate in improving corrosion resistance and paint adhesion of the magnesium alloys.
The acid cleaner primarily includes nitric acid and fluoride. The chemical conversion-processing agent primarily includes a VB metal compound and a pH-adjusting agent. The coupling agent also further includes a solvent, a solvent aid and water.
The present invention discloses a method for treating magnesium alloys by chemical conversion, which can improve corrosion resistance and paint adhesion of the magnesium alloys, and produce an admirable appearance without additional paint on surfaces thereof. A wide variety of colors from dark green to gold can also be formed.
In general, surface treatment of metals, such as iron, aluminum, zinc, cadmium and magnesium, includes cleaning with alkali or acid cleaner. Whereby following processes such as painting can be applied easily and the products have better properties and appearances. This process is known as metal chemical conversion. The chemicals used in the present invention are suitable for all magnesium alloys, particularly for AZ31, AZ61 and AZ91. The surface treatment of the present invention primarily includes steps of:
(1) cleaning the surface of the magnesium alloy with an alkali cleaner to degrease contamination;
(2) rinsing the magnesium alloy with water to eliminate the alkali cleaner;
(3) pickling the magnesium alloy with acid cleaner to eliminate oxides and metal impurities on the surface;
(4) rinsing the magnesium alloy with water to eliminate the acid cleaner;
(5) treating the surface of the magnesium alloy with a non-chromate agent to form a slightly coarse surface film for corrosion resistance as well as enhancing adhesive of paint; and
(6) treating the surface with organometallic Ti or Silane to reinforce corrosion resistance and paint adhesion of the surface film.
Each of the above steps can be applied individually or associatively to various painting technologies.
Alkali cleaning is usually the first step of surface treatment, in which an alkali cleaner is applied to remove oil contamination on the surface of magnesium alloys. The alkali cleaner can be prepared by users or selected from commercial productions such as Turco-4215-NC-LT. The temperature can be set between 40xc2x0 C. and 60xc2x0 C., preferably between 45xc2x0 C. and 55xc2x0 C. The time for treatment is about 5 to 10 minutes, with the pH value preferably kept between 9 and 11.
The alkali cleaner generally includes an alkali as the main component and a surfactant, wherein the alkali component is usually Na2CO3, NaOH or Na3PO4, and the surfactant can be the additives mentioned in U.S. Pat. No. 4,370,173. Additionally, chelating agent such as sodium gluconate can be used to remove magnesium or calcium ions, and therefore reduces water hardness, enabling easy oil elimination by the surfactant. The alkali cleaner is suitable for removing impurities that are not strongly attacked to the surface of the magnesium alloy. As for the oxides or impurities with stronger adhesion, the acid pickling has to be carried out after water rinse. By means of alkali cleaning, water rinsing, acid pickling, and water rinsing again, a clean surface of magnesium alloy can be obtained for chemical conversion. The rinsing water is preferably deionized water in order to prevent the surface film from being deteriorated by undesired agents such as chlorine ions.
The acid pickling agent in the present invention is prepared with NH4HF2 (10-25 g/L) and nitric acid (30-50 c.c./L) at room temperature (RT), in which the magnesium alloy is steeped for 30-60 seconds. Furthermore, adding a fluoro-surfactant to the acid pickling agent could promote activating effects and prevent magnesium alloy from etching greatly by the acid pickling agent. For water rinsing after acid pickling, flowing deionized water is preferred, and the magnesium alloy is preferably dipped therein for at least 5 minutes to completely remove the acid pickling agent.
After alkali cleaning and acid pickling, the surface of the magnesium alloy is active, and should be brought to the chemical conversion process as soon as possible to avoid surface oxidization. In the present invention, the chemical conversion processing agent primarily includes VB or rare-earth metal salts, for example, V2O5, Na2V2O4, NaVO3 and CeCl3, etc. The chemical conversion-processing agent usually has a concentration between 10 g/L and 80 g/L, preferably between 40 g/L and 60 g/L, and has a pH between 3 and 10, preferably between 6 and 8. The time for the chemical conversion process is about 1-10 minutes, preferably 3-5 minutes, and the temperature is controlled between room temperature and 60xc2x0 C., preferably between 35xc2x0 C. and 45xc2x0 C. A wetting agent, for example, TX-100 (Merck), CO-720 (Aldrich) or dodecyl sodium sulfate (Aldrich) is used to enhancing reaction rates and roughing the surface of the alloy, which is instrumental in adhesion of paint thereon. Addition of the pH-adjusting agent varies the film surface color, for example, low pH for greenish color, medium pH for golden color, and high pH for dark green color. The pH-adjusting agents are such chemicals as acetic acid, formic acid, NaOH, ammonia and ethylene diamine, etc.
In order to have superior corrosion resistance and painting adhesion on the surface film, sealing treatment is involved after the chemical conversion. The sealing agent can be a silane or titanium coupling agent, for example, vinyl silane, glycidoxy silane and mercapto silane, and the commercial productions include Dow Corning Z-6040, Z-6032, Z-6020, etc., or KEN-REACT KRTTS, KR44, KR38S, etc. for the titanium coupling agents. The above agents are prepared with 18 Mxcexa9 pure water, alcohols, etc.
The method for preparing the coupling agent may include:
1. mixing the coupling agent (x)ml with an equivalent amount of alcohol;
2. adding pure water (y)ml=(x/m.w. of the coupling agent)xc3x97xc3x9718 into the above mixture;
3. resting the above solution for 20 minutes and then diluting to 10% with a solvent aid.
After surface treatment, samples are preferably left for 24 hours before painting to achieve better corrosion resistance and adhesion.